The horse lentivirus, equine infectious anemia virus, (EIA) is a vitus closely related to the human immunodeficiency virus. EIA may be useful as a lentivirus model for the study of macrophage infection and viral latency -- tow important components of early acquired immunodeficiency syndrome. Three projects are currently ongoing to study how latency is overcome and to study the role the peripheral blood monocyte plays in EIA. Reconstruction of a molecular clone of EIA is in progress. This clone is not infectious, at least in part because of a stop codon in the envelope gene. Using site directed mutagenesis, the stop codon has been repaired and the molecular clone is currently being tested for infectivity in both tissue culture cells and horse macrophages. In parallel with this work, stop codons have been separately introduced into each of the three small open reading frames (sORG) found in EIA and analysis of the role these ORFs play within a full length clone is ongoing. The S1ORF has been shown to encode a transactivating proteins (tat). The introduction of a stop codon into the tat coding sequence in EIA resulted both in a loss of transactivating activity and a loss of the expression of viral antigens in tissue culture cells. Similar studies are ongoing with primary horse macrophages. These results suggest that similar to HIV tranactivation may be required for viral protein expression and infectivity. Functionally analysis of the other two sOrf is ongoing. In another project, we are studying latency and reactivation of the virus in an EIA-seropositive mare. This mare has never been clinically ill with EIA. No virus has been detected in her peripheral blood mononuclear cells (PBMCs) or in serum. Viral reactivation is possible by extensive steroid treatments of the animal. During viral reactivation, we plan to follow the molecular development of viral expression as well as the clinical course of the disease. Upon cessation of steroid treatments, clinical manifestations of EIA should abate and it may be possible to study reentry into a latent state. Another outgrowth of this study will be the isolation of a new field isolate of EIA. To date, only two field isolates have been characterized. Unlike the sequence heterogeneity found in different isolate of HIV, the two EIA isolate are virtual identical . This, to begin to understand if heterogeneity exists between stains of EIA isolation, molecular analysis of a new field strain will be performed. A third project involves the development and characterization of peptide specific antibodies. Currently, few protein or peptide specific reagents are available for EIA. These antibodies are being developed for use in Western blots, immunopreciptations and cell staining assays.